java 如何读取 jar 中的文件资源和路径

问题描述：项目需要将生成的文本存储在运行环境中，当我用相对路径 safe/certificate.pem存储时，出现了两种情况：

第一种：用idea启动项目
这时生成的文本会在编译后的 target.classes下创建对应目录，这时用百度上千篇一律的如下代码可以成功读取。问题的关键在于直接用idea启动的不是项目编译后的jar包 !

 public String getCert() {
        java.net.URL fileURL = this.getClass().getResource("/safe/certificate.pem");
        if (fileURL == null) {
            return "没有读取到证书！";
        }
        File file = new File(fileURL.getPath());
        long filelength = file.length();
        byte[] filecontent = new byte[(int) filelength];
        try {
            FileInputStream in = new FileInputStream(file);
            in.read(filecontent);
            in.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return new String(filecontent);
    }

第二种：用cmd 运行jar 包启动项目
通过jar运行的项目，会在jar包存储路径中生成jar同名文件，此时相对路径safe/certificate.pem生成的文本会在 jar 同名文件目录下创建对应目录，如下图蓝色框内容，这时通过上述的代码this.getClass().getResource(文件名)是获取不到文件的，会是null
解决办法

1. 得到 jar 文件路径

 String path = this.getClass().getProtectionDomain().getCodeSource().getLocation().getFile();
 
 //path : file:/C:/Users/Mr.Tan/Documents/IT/work/HB/danger-backend/danger-client/cipher-client/target/cipher-client.jar!/BOOT-INF/classes!/

2. 解决可能存在的乱码

 String path = this.getClass().getProtectionDomain().getCodeSource().getLocation().getFile();
 try {
            path = java.net.URLDecoder.decode(path, "UTF-8");
            log.info("utf-8 path: " + path);
        } catch (java.io.UnsupportedEncodingException ex) {
            log.error(ex.getLocalizedMessage());
        }
        
// utf-8 path: file:/C:/Users/Mr.Tan/Documents/IT/work/HB/danger-backend/danger-client/cipher-client/target/cipher-client.jar!/BOOT-INF/classes!/        

3. 获取绝对路径

String path = this.getClass().getProtectionDomain().getCodeSource().getLocation().getFile();
        log.info("path: " + path);
        try {
            path = java.net.URLDecoder.decode(path, "UTF-8");
            log.info("utf-8 path: " + path);
        } catch (java.io.UnsupportedEncodingException ex) {
            log.error(ex.getLocalizedMessage());

        }
        java.io.File jarFile = new java.io.File(path);
        log.info("jarFilepath: "+jarFile.getAbsolutePath() );

//jarFilepath:  C:\Users\Mr.Tan\Documents\IT\work\HB\danger-backend\danger-client\cipher-client\target\file:\C:\Users\Mr.Tan\Documents\IT\work\HB\danger-backend\danger-client\cipher-client\target\cipher-client.jar!\BOOT-INF\classes!

4. 获取上一级文件

java.io.File parent = jarFile.getParentFile();
        log.info("parent: "+parent.getName());
        log.info(parent.getAbsolutePath());

// parent: BOOT-INF
// C:\Users\Mr.Tan\Documents\IT\work\HB\danger-backend\danger-client\cipher-client\target\file:\C:\Users\Mr.Tan\Documents\IT\work\HB\danger-backend\danger-client\cipher-client\target\cipher-client.jar!\BOOT-INF

参考代码
将生成的文件存到获取的绝对路径中，读取时从绝对路径读，将文件存在filet目录的上级目录，

	/**
     * 获取jar包中的绝对路径
     *
     * @return java.lang.String
     * @author luce
     * @date 2021/9/7 15:24
     */
    private String acquireJarPath() {
    	//jar包编译后的相对路径
        String path = this.getClass().getProtectionDomain().getCodeSource().getLocation().getFile();
        //消除乱码
        try {
            path = java.net.URLDecoder.decode(path, "UTF-8");
        } catch (java.io.UnsupportedEncodingException ex) {
            log.error(ex.getLocalizedMessage());
        }
        //根据路径获取目标文件
        java.io.File jarFile = new java.io.File(path);
        //获取文件的绝对路径
        String jarFilepath = jarFile.getAbsolutePath();
        //输出内容：//jarFilepath:  C:\Users\Mr.Tan\Documents\IT\work\HB\danger-backend\danger-client\cipher-client\target\file:\C:\Users\Mr.Tan\Documents\IT\work\HB\danger-backend\danger-client\cipher-client\target\cipher-client.jar!\BOOT-INF\classes!
        //我们需要得到 file 的上级目录
        int end = jarFilepath.indexOf("file");
        if (end > 0) {
            return jarFilepath.substring(0, end);
            //输出结果：C:\Users\Mr.Tan\Documents\IT\work\HB\danger-backend\danger-client\cipher-client\target\
        }
        return jarFilepath + "\\";
    }


	/**
     * 存证书文件
     *
     * @param x509Certificate 证书
     * @param certStoragePath 存储路径 ->  C:\Users\Mr.Tan\Documents\IT\work\HB\cipher\target\safe\certificate.pem
     * @author luce
     * @date 2021/8/31 15:27
     */
    public static void createCert(X509Certificate x509Certificate, String certStoragePath) throws CertificateEncodingException {
    	//先删除，预防在原有文本中续写
        FileUtil.del(acquireJarPath() + "safe/certificate.pem");
        //在指定目录生成证书文件
        FileUtil.newFile(certStoragePath);
        //证书标题
        FileUtil.appendUtf8String(BEGIN_CERT, certStoragePath);
        //分割线
        FileUtil.appendUtf8String(System.getProperty("line.separator"), certStoragePath);
        //转Base64码的证书
        FileUtil.appendUtf8String(Base64Encoder.encode(x509Certificate.getEncoded()), certStoragePath);
        //分割线
        FileUtil.appendUtf8String(System.getProperty("line.separator"), certStoragePath);
        //尾文
        FileUtil.appendUtf8String(END_CERT, certStoragePath);
    }


	/**
     * 读取证书
     *
     * @return java.lang.String
     * @author luce
     * @date 2021/9/3 10:09
     */
    public String getCert() {
        File file = new File(acquireJarPath() + "safe/certificate.pem");
        //读取的路径：C:\Users\Mr.Tan\Documents\IT\work\HB\danger-backend\danger-client\cipher-client\target\safe/certificate.pem
        long filelength = file.length();
        byte[] filecontent = new byte[(int) filelength];
        try {
            FileInputStream in = new FileInputStream(file);
            in.read(filecontent);
            in.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return new String(filecontent);
    }